Porous anodic alumina (PAA) films are films generated by electrochemical oxidation of aluminum under selective conditions (electrolyte, temperature and voltage). These films have a unique morphology of a honeycomb array of channels, several nanometers in diameter and several microns in length, which are perpendicular to the surface of the film. At the alumina-aluminum interface however there is a non-porous undulated alumina barrier layer several nanometers thick. Since the pore size, the pore length, the inter-pore distance, and the barrier layer thickness can all be controlled by the anodization conditions, PAA films have attracted a lot of interest as a nanotechnology tool. PAA films have found applications as filters, collimators, as templates for nano-patterning and nanowire growth, and as photonic bandgap materials.
PAA films have several disadvantages associated with them. These disadvantages have precluded the use of PAA films in a wider range of applications. Free-standing anodic films are extremely fragile and cannot sustain stress. Even when the film is attached to the aluminum substrate, the film may fracture since aluminum is a soft metal. Such uniform, small-feature and controllable porous structures have been successfully grown only on aluminum, and not on any other substrate. The growing porous film is separated from the underlying metallic aluminum by a scalloped layer of oxide, known as the barrier layer. The barrier layer prevents electrical contact to be established with the bottoms of the pores of the film.
The conventional way of fabricating the PAA films starts with an aluminum sheet that goes through several steps of mechanical and electrochemical polishing. Once the surface roughness of the sheet is down to the sub-micron level, the metal is anodized in an acidic bath and the porous alumina is obtained. The quality of the starting anodic alumina is usually low in terms of the ordering and uniformity of the pores. Therefore, this initial film is typically etched away and a new PAA film is grown under the same or similar anodization conditions. The pores cannot be provided all the way through the aluminum, since an electrical path through the aluminum is necessary to perform the anodization, and the aluminum substrate functions as an electrode for the anodization process. In order to obtain a PAA membrane in which the pores run completely through the film and are open (and accessible) on both sides, it is necessary to etch away the metallic aluminum sustaining the oxide and subsequently also to etch away the barrier layer, or to detach the membrane from the aluminum substrate by one of the available methods to do so.
In practice, this process has several disadvantages associated with it. The mechanical polishing steps introduce imperfections and contamination, limit the active area of the film, and limit the throughput of the process. Another disadvantage with the prior art process is that after the removal of the sustaining metal, the free-standing PAA film is very brittle and is hard to manipulate effectively. Further, during the etch steps, the surface topography of the film is degraded thereby affecting the optical properties of the film and its use as a mask.
It would, therefore, be desirable to provide a method which allows for the fabrication of PAA films on a wide variety of substrates. When a rigid substrate is used, the resulting anodic film is more tractable, easily grown on extensive areas in a uniform manner, and can be manipulated without danger of fracturing. It would be further desirable to provide the film on patterned and non-planar surfaces. It would still further be desirable to provide the PAA film missing the barrier layer (partially or completely) such that the bottom of the pores can be readily accessed electrically such as by a conducting layer on the substrate. Having such a film, an array of nanowires perpendicular to the surface of the film can be deposited into the pores.
It would be further desirable to provide the PAA film on a patterned conducting layer such that the resulting anodic film can be provided with one set of pores filled with one type of nanowire material (e.g. n-type material) and another set of pores provided with a different nanowire material (e.g. p-type). It would be further desirable to provide the PAA films missing the barrier layer on a patterned conducting layer such that pores, or nanowires within the pores, can be electrically addressed independently from each other. It would further be desirable to provide the PAA templates such that multiple stages of the templates can be built, and can be stacked to form a multi-stage device.